Minimal change disease (MCD) is the most common nephrotic syndrome in children, and can be associated with marked morbidity. Currently the etiology of MCD is unknown, but it has been hypothesized to be due to a circulating factor, possibly released by activated T cells, that induces podocyte dysfunction, resulting in an alteration of glomerular permeability with the development of nephrotic proteinuria. Our group has hypothesized that MCD is mediated by a phenotypic change in podocytes in which they express CD80, which is then shed into the urine as the podocyte undergoes shape change. In preliminary data we have found elevated levels of urinary CD80 in patients with active (relapsing) MCD and that it is absent in normal individuals, in MCD in remission, and in proteinuric subjects with focal segmental glomerulosclerosis. We have also identified CD80 in podocytes of limited numbers of renal biopsies of subjects with MCD. We have also developed an in vitro model of MCD in which we can activate toll-like receptors present on cultured human podocytes and show that it activates interferon and NF:B dependent pathways with the expression of CD80, shape change (actin rearrangement) and loss of synaptopodin, and we can rescue this with corticosteroids (which are used clinically to treat minimal change disease). In this application we will first perform cell culture studies to evaluate the potential role of serum and peripheral blood mononuclear leukocyte supernatants from subjects with and without MCD in stimulating CD80 expression in vitro, to determine the cellular signaling pathways by which Toll-like receptor ligands stimulate CD80 in podocytes in vitro, and to develop an animal model of MCD based on these findings. Second, we will perform additional clinical studies to determine the specificity, temporal excretion and podocyte expression of CD80 in MCD compared to other glomerular disorders. We will also examine the urinary excretion and podocyte expression of CTLA-4 for which we hypothesize is the factor that autoregulates the CD80 response. It is our hope that these studies may elucidate not only the cause of minimal change disease but also reveal new strategies for treatment and prevention.